As amounts of information on networks increase, the WDM (Wavelength Division Multiplexing) system is gaining popularity. The WDM multiplexes and transmits a plurality of optical signals using a plurality of wavelengths. For this reason, in the WDM system, the transmission capacity increases as the number of multiplexed wavelengths increases.
In the WDM system, all or some of optical nodes have an OADM (Optical Add-Drop Multiplexer). An optical add-drop multiplexer can insert an optical signal into an optical transmission path with respect to wavelength and branch an optical signal from the optical transmission path with respect to wavelength. In other words, an optical add-drop multiplexer has an optical add device (or, an optical add function) to add an optical signal of a desired wavelength to a WDM optical signal, and an optical drop device (or, an optical drop function) to drop an optical signal of a desired wavelength from a WDM optical signal. Meanwhile, an optical add-drop multiplexer that adds and/or drops an optical signal of a desired wavelength is sometimes called an ROADM (Reconfigurable OADM).
As a related art, an optical transmission apparatus described below has been proposed. An optical transmission apparatus has at least two variable wavelength selective filters including a first variable wavelength selective filter that performs drop/add operations for some signals among optical signals to be dropped/added, and a second variable wavelength selective filter that performs drop/add operations for optical signals that are not selected by the first variable wavelength selective filter among the optical signals to be dropped/added (for example, Japanese Laid-open Patent Publication No. H11-289296).
As another related art, an optical add-drop multiplexer described below has been proposed. An optical add-drop multiplexer uses a droptype AOTF (Acousto-Optic Tunable Filter) as a configuration to remove an optical signal of the same wavelength as the wavelength of an added optical signal from a “through” signal. In the drop type AOTF, RF signals are input so that all wavelengths are constantly selected, and only for the wavelength to be rejected, corresponding RF signal is stopped. Accordingly, an optical signal of the wavelength for which corresponding RF signal is stopped is not selected by the drop type AOTF, and the optical signal does not pass through (for example, Japanese Laid-open Patent Publication No. 2005-348270).
As yet another related art, an ROADM terminal apparatus that adds and drops an optical signal of a given wavelength component has been proposed. The ROADM terminal apparatus has an optical switching unit for transmitting and receiving a given wavelength component of an optical signal to/from another ROADM terminal apparatus, an OSC control information generation unit that generates OSC (Optical Supervisor Channel) control information to drop/add an optical signal, and an OSC control information input/output I/F for transmitting and receiving the OSC control information to/from another ROADM terminal apparatus (for example, Japanese Laid-open Patent Publication No. 2009-206707).
As yet another related art, a wavelength selective switch that may perform add/drop functions for all input channels and has a high degree of freedom has been proposed. The wavelength selective switch includes an optical demultiplexing unit that divides an input optical signal with respect to wavelength corresponding to each channel, and selects and outputs an optical signal divided from the input optical signal or an optical signal input through an additional port, an optical deflecting unit that deflects an optical signal of each channel received from the optical demultiplexing unit individually by current supply or voltage application, and an optical multiplexing unit that outputs the optical signal for each channel to a specific output port by the deflection by the optical deflecting unit (for example, Japanese Laid-open Patent Publication No. 2009-145869).
It is preferable that an optical add-drop multiplexer has a configuration with which the wavelength path may be set or changed flexibly. For example, it is preferable that an optical add-drop multiplexer is Colorless, Directionless and Contentionless.
“Colorless” indicates a configuration or function with which a desired wavelength may be input to a desired port of the optical add-drop multiplexer, and a desired wavelength may be output from a desired port. “Directionless” indicates a configuration or function with which, in a configuration in which the optical add-drop multiplexer has a plurality of incoming/outgoing routes, an optical signal from a terminal may be guided to a desired outgoing route, and an optical signal from each incoming route may be guided to a desired terminal. “Contentionless” indicates a configuration or function to avoid contention between optical signals of the same wavelength within the optical add-drop multiplexer.
However, in a conventional optical add-drop multiplexer, it is difficult to set up or change a wavelength path flexibly. That is, in a conventional optical add-drop multiplexer, it is difficult to set up a desired wavelength path. For example, in a conventional optical add-drop multiplexer, when switching from a state to output an optical signal transmitted from a terminal to a first outgoing route into a state to output the optical signal to a second outgoing route, it is required to change an input port to which the optical fiber from the terminal is connected.